Apparatus enabling use of a reference diode to compare against a device under test in relative amplitude and phase measurements

ABSTRACT

Embodiments of the invention include methods and devices for determining a phase angle offset between a phase angle of a local oscillator relative to a phase angle of a signal input of a Device Under Test (DUT). Some embodiments include a laser source and an optical phase adjustor, which may be embodied by a loop stretcher structured to controllably stretch a length of fiber optic cable, driven by a phase adjust driver. In other embodiments the phase angle offset information is conveyed to an oscilloscope for internal compensation.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims benefit from U.S. Provisional Application62/083,148, filed Nov. 21, 2014, entitled TEST AND MEASUREMENT DEVICE,and also claims benefit from U.S. Provisional Application 62/211,614,filed Aug. 28, 2015, entitled APPARATUS FOR MEASURING FREQUENCY RESPONSEOF AN INTEGRATED COHERENT OPTICAL RECEIVER FRONT END USINGEQUIVALENT-TIME SAMPLING. This application is also related to U.S.patent application Ser. No. 14/873,997, entitled TEST AND MEASUREMENTDEVICE FOR MEASURING INTEGRATED COHERENT OPTICAL RECEIVER. The contentsof all of the applications referred to in this paragraph areincorporated by reference herein.

FIELD OF INVENTION

This disclosure generally relates to test and measurement devices, and,more particularly, to an apparatus that enables testing the frequencyresponse of an Integrated Coherent Optical Receiver (ICR) usingoscilloscopes.

BACKGROUND

Optical communication systems transmit data using electromagnetic lightsignals in optical fiber and/or free space (for example, building tobuilding, ground to satellite, satellite to satellite, etc.). Theelectromagnetic carrier wave is modulated to carry the data. Opticalcommunication in optical fiber typically involves: generating theoptical signal, relaying the signal on an optical fiber (includingmeasures to reduce/mitigate attenuation of, interference with and/ordistortion of the light signal), processing a received optical signal,and converting the signal into a useful electrical signal. Transmitterscan be semiconductor devices such as laser diodes, producing coherentlight for transmission. A number of receivers have been developed forprocessing a transmitted lightwave optical signal to provide processedoptical signal input(s) to one or more photodetectors, which convertlight into electricity.

A coherent receiver, such as an Integrated Coherent Optical Receiver(ICR), converts a modulated optical signal into four electrical signalscorresponding to an “in-phase” (I) and “quadrature” (Q) optical signalcomponents of the two optical polarization states, vertical andhorizontal. These components can be processed to recover the opticallytransmitted data regardless of modulation type. Thus, the four outputelectrical signals from the ICR carry all or nearly all of theinformation conveyed by the optical signal.

Testing an ICR presents a special challenge in that the output stage isa balanced detector pair often followed by a differential amplifier withdifferential outputs. The fact that there are four differential outputs(I and Q each for X and Y polarizations), compounds the difficulty. Asimple coherent receiver is composed of a local-oscillator laser, anoptical coupler, and one or more photodetectors that can be in a“balanced” configuration that cancels photocurrents and eliminates DCterms and the related excess intensity noise.

The balanced detection and differential amplification of the ICR ensurethat any signal put into only the signal port or only the LocalOscillator (LO) port of the ICR will be rejected unless it is possibleto block one of the photodiodes to break the balanced detection.Although early versions of ICRs allowed physical access to interrupt alight signal and thereby break the balanced detection, this is notpossible on modern integrated components, which are instead typicallyintrinsically sealed. Getting any meaningful signal out of the ICRtherefore requires both a signal and a LO input. This requirement cancomplicate some desired measurements to be performed on a Device UnderTest (DUT), where the optical LO input must be phase coherent with thetest signal input.

Because the ICR requires both a signal and a local oscillator input toprovide meaningful output, the frequency and phase relationship betweenthe two input signals are important. While it is simplest to split theLO and Signal lasers and then connect them to a reference coherentreceiver front end and a Device Under Test (DUT), the separate fiberpaths required by this configuration can introduce an unknown phasedifference between the input signals.

Embodiments of the present invention determine, correct for, and/orcontrol a phase difference between the local oscillator signal and thetest signal input to a DUT. This ability can be used to restore thephase coherence needed for certain desired performance tests of the DUT.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference is made to embodiments of the invention, examples of which maybe illustrated in the accompanying figures. These figures are intendedto be illustrative, not limiting. Although the invention is described inthe context of these embodiments, it should be understood that thisdescription is not intended to limit the scope of the invention to theseparticular embodiments.

FIG. 1 is a block diagram illustrating components of an IntegratedCoherent Optical Receiver to be tested using embodiments of theinvention.

FIG. 2 is a block diagram illustrating components of a precise phaseadjusting system that may be used in implementing embodiments of theinvention.

FIG. 3 is a block diagram illustrating components of a precise phaseadjusting system that may be used in implementing embodiments of theinvention.

FIG. 4 is a block diagram illustrating components of a precise phaseadjusting system that may be used in implementing embodiments of theinvention.

FIGS. 5A and 5B illustrate filters used in embodiments of the invention.

FIG. 6 is a graph illustrating phase vs. frequency data before and afterdeskewing according to embodiments of the invention.

FIG. 7 is a block and schematic diagram illustrating a phase adjustdriver according to embodiments of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description will refer to one or moreembodiments, but the present invention is not limited to suchembodiments. Rather, the detailed description and any embodiment(s)presented are intended only to be illustrative. Those skilled in the artwill readily appreciate that the detailed description given herein withrespect to the Figures is provided for explanatory purposes as theinvention extends beyond these limited embodiments.

Certain terms are used throughout the description and claims to refer toparticular system components. In the following discussion and in theclaims, the terms “including” and “comprising” are used in an open-endedfashion, and thus should be interpreted to mean “including, but notlimited to . . . . ” Phrases such as “coupled to” and “connected to” andthe like are used herein to describe a connection between two devices,elements and/or components and are intended (unless otherwise restrictedspecifically) to mean physically, optically and/or electrically eithercoupled directly together, or coupled indirectly together, for examplevia one or more intervening elements or components or via a wireless orother connection, where appropriate. The term “system” refers broadly toa collection of two or more components and may be used to refer to anoverall system (e.g., a communication system, a receiving system, atesting system, a computer system or a network of such devices/systems),a subsystem provided as part of a larger system, and/or a process ormethod pertaining to operation of such a system or subsystem.

Probably the most important characteristics to be measured of a DeviceUnder Test (DUT) are the shapes of its amplitude and phase response atvarious frequencies. A difficulty with measuring these characteristics,as mentioned above, is that the ICR used for testing produces nomeaningful output unless two inputs are present, a signal input and alocal oscillator (LO) input. For the accurate measurement of theamplitude and phase response, the two inputs to the ICR must be phasecoherent. This requirement is addressed in some embodiments of thepresent invention by the inclusion of a phase controller including anoptical phase-locked-loop. Other embodiments use various methods ofevaluating and correcting the phase relationship. Embodiments of theinvention restore phase coherence by using an LO having a tunable phaseadjust, or by providing a measurement of the amount by which the LO andSignal are out of phase. This latter quantity can be used in subsequentprocessing to correct the phase and amplitude response of the DUT.

As seen in FIG. 1, a generalized optical signal processor 100, which canbe used as an optical signal receiver or an optical testing device, forexample, accepts an unprocessed data signal beam 110 at a SIG input aswell as a local oscillator beam 120 at an LO input. The optical signalprocessor 100 may also be referred to as a Device Under Test (DUT). Insome embodiments the beams 110, 120 may be laser beams, but embodimentsof the invention also work in typical RF frequencies. The laser used forthe local oscillator 120 can be any suitable laser source and type (forexample, continuous wave, pulsed, etc.). A Polarized Beam Splitter (PBS)130 splits the beams 110, 120 into two channels. As noted below, and aswill be appreciated by those skilled in the art, the local oscillator120 should preferably be oriented so that sufficient reference localoscillator power is available downstream along any needed polarizations.In some embodiments discussed herein, such polarizations may be referredto as “X” and “Y” polarizations (as well as other orientationrepresentations such as “vertical” and “horizontal), etc.), though suchnomenclature only serves to describe the relative polarizationorientations, as do references to 45° offsets and/or axes relative tosuch horizontal and/or vertical polarization orientations. Those skilledin the art will appreciate that many equivalent structures, apparatus,etc. are available once the relative polarization schemes, etc. areknown.

An X-channel optical mixer 140 combines the signal 110 and LO inputs 120of the X-channel output of the PBS 130 to produce a differentialin-phase (I) output on outputs 141, 143, and a differential quadrature(Q) output on outputs 145, 147. These outputs are directed tophotodetectors, such as photosensitive diode pairs 142, 144, and 146,148. The photodiodes 142, 144, 146, 148 of the signal processor 100 maybe coupled to an amplifier, such as transimpedance amplifiers 151, 155,and/or other components well known to those skilled in the art and usedto extract data from the signal beam 110. Embodiments of the inventionoperate even in absence of other components coupled to the photodiodes142, 144, 146, 148. Current monitors 182, 184 may be coupled to theoutputs of the photosensitive diode pairs 142, 144, respectively. Such amonitor allows each photodiode, referred to as P and N, for positive andnegative, from each channel, to be monitored. For example, the currentmonitor 182 monitors the P photodiode 142 of the XI channel, while thecurrent monitor 184 monitors the N photodiode 144 of the XI channel.Although not illustrated, each of the channels XQ, YI, and YQ mayinclude current monitors for the P and N photodiodes, so that each ofthe outputs of the photodiodes in each channel may be individuallymonitored.

Similarly, a Y-channel optical mixer 160 combines the signal 110 and LOinputs 120 of the Y-channel to produce a differential in-phase (I)output on outputs 161, 163, and a differential quadrature (Q) output onoutputs 165, 167. These outputs are directed to photodetectors, such asphotosensitive diode pairs 162, 164, and 166, 168. The photodiodes 162,164, 166, 168 of the signal processor 100 of FIG. 1 are coupled totransimpedance amplifiers 171, 175, respectively, but such amplifiersare not needed to practice embodiments of the invention.

As mentioned above, it is difficult to test the frequency responses ofthe photodiodes of the X and Y channels without having physical accessto block light to one of the pairs of diodes. It is also very difficultto test the frequency responses of the photodiodes without an ability toprecisely control the frequency of the local oscillator beam 120,especially a local oscillator that oscillates at the very high opticalcarrier frequencies, such as 192 THz. Another problem exists in that thefiber carrying the signal 110 may have a different length than the fibercarrying the local oscillator 120, which makes it difficult to matchphases of the signal and local oscillator. Therefore, embodiments of theinvention provide an apparatus and method to hold the phase of the localoscillator extremely stable, and well matched to the phase of thesignal, as well as being controllable, to facilitate measurement of thefrequency responses of the photodiodes.

Using the arrangement shown in FIG. 2, a delay (or phase) locked loopcan be used to drastically reduce the phase wander between the referenceand DUT paths. In addition, the methods described herein lend themselveswell to the use of a reference photodiode rather than a referencecoherent receiver front end, drastically reducing the cost of thetesting setup. The technique can be extended for use with equivalenttime oscilloscopes.

One embodiment creates a reference signal by combining two lasers in aPhase Maintaining (PM) coupler. With reference to FIG. 2, a laser 240generates an LO signal that is split in a PM splitter 241. One output ofthe PM splitter 241 becomes the LO input for a DUT 220. The Signal inputfor the DUT 220 is generated by a laser 210. A difficulty in measuringthe performance of the DUT 220 is providing the LO at the DUT 220 at thesame phase with respect to the Signal, generated by the laser 210, thatis seen at the reference detector 216. Embodiments of the inventionaddress this difficulty by precisely controlling the phases of the LOand the Signal relative to each other. First, the output of the laser240 is split in a PM fiber splitter 241. One output of the fibersplitter 241 becomes the LO input to the DUT, as described above. Theother output of the fiber splitter 241 passes through a phase adjustloop 230 to a 2×2 PM fiber coupler 212. The fiber coupler 212 firstduplicates the Signal output from laser 210 on both outputs, one goingto the reference detector 216, and the other going to the Signal inputfor the DUT. The phase of the outputs of the fiber coupler 212 iscontrolled by the phase of the output of the laser 240, that was splitin the fiber splitter 241, but only after the output passes through aphase or delay locked loop 230. The error signal to control the loop 230is the beat signal between the light output from the first laser 210that travels through the PM fiber coupler 212 to the reference detector216, and the light output from the first laser 210 that travels directlyto the DUT 220. By adjusting the phase of the latter to match theformer, an identical optical test signal is created at both thereference detector 216 and the Signal input of the DUT 220, allowing anoscilloscope 250 to monitor the desired signals.

The phase adjust driver 232 controls the phase adjust loop 230 to selectparticular phases and provide a stable phase reference of the localoscillator relative to the signal. The level to which the phase adjustdriver 232 is driven is related to its input, called an error input, forreasons described below. This error input is generated in what isreferred to as DC loop gain block 234. The error input from the loopgain block 234 to the phase adjust driver 232 is a selected one or pairof current monitors from the DUT 220. A monitor selector 236 controlswhich of the monitored channel outputs of the DUT 220 is provided as theerror input of the phase adjust driver 232 to cause the phase adjustloop 230 to shift the phase of the LO input relative to the Signal inputof the DUT. For example, the error input to the phase driver could bethe current monitors 182, 184 of the XI channel as illustrated in FIG.1, or any of the other monitored channels of the optical signalprocessor 100 of FIG. 1. The error input to the phase adjust driver 232can perform both a dc-bias function as well as the phase adjustfunction. In this way it can completely take over for the user biascircuitry of prior solutions, eliminating the need for any suchinteraction between the test system and the bias circuitry of the user.In addition, the phase adjust driver 232 can also be used to measurephotocurrents of the DUT 230 outputs to determine DC Common ModeRejection Ratio (CMRR) for example.

The phase adjuster, such as the phase adjust loops illustrated in FIG. 2may be embodied by a piezo phase stretcher. A piezo phase stretcher hasloops of fiber optic cable looped around a piezoelectric element.Energizing the piezoelectric element causes the element to expand, whichlengthens the path of the fiber optic material looped around it.Lengthening the path changes the distance the light travels through thepath. Lengthening only one of the paths, Signal or LO, allows the phaseof one signal to be adjusted relative to the other. In one embodimentthe phase adjust loop may allow light traveling through the loop to beadjusted extremely accurately, such as on the order of tenths orhundredths of a picosecond. In other words, by using the phasestretcher, the fiber length that the laser light travels through may belengthened to cause the light traveling through it to take, for example,0.1 ps longer than the fiber in its non-stretched length. This changesthe relative phase of the light signals between the LO and the Signalinputs of the DUT. Therefore, driving the phase adjuster gives phaseprecise phase control to the system. The length of both laser paths fromthe laser for both the Signal and LO may be set up to be similar lengthto one another, for example within one meter. The phase adjust loop insome embodiments may be an OptiPhase PZ1-PM4-APC-E-155B, and may have afiber length of 12.36 m, for instance. Of course, other methods ofadjusting the phase other than using a piezo phase stretcher arepossible without deviating from the spirit of the invention.

Providing a signal to a DUT that is phase stable and controllably lockedrelative to the LO in a known relationship allows the same results to beobtained from the DUT as if the balanced detection were able to bephysically blocked, as in previous solutions, as proven by the theory ofoperation shown below.Signal Field:

=E ₁ e ^(jω) ¹ ^(t) =E ₁₀ e ^(jθ) ¹ e ^(jω) ¹ ^(t)  Equation (1):LO Field:

=E ₂ e ^(jω) ¹ ^(t) =E ₂₀ e ^(jθ) ² e ^(jω) ¹ ^(t)  Equation (2):

For simple amplitude modulation of E₁₀ with a balanced drive, the outputof the DUT for a particular polarization will be proportional to:I: E₁₀E₂₀ cos(θ₁−θ₂)  Equation (3):Q: E₁₀E₂₀ sin(θ₁−θ₂)  Equation (4):

A phase-locked loop with the Q dc level output provided as the errorinput can then be used to drive the θ₁−θ₂ to zero, providing an outputon the I-channel that is proportional to E₁₀, which gives the responseof the positive diode of the differential pair, such as the diode 142 ofthe differential pair of diodes 142, 144 of FIG. 1. Driving θ₁-θ₂ to πgives −E₁₀, which is the response of the negative diode, such as diode144. This response signal may be selected in the phase adjust driver bychanging the sign of the gain. Any of the four photodiode pair outputsfrom the DUT may be selected to be the error input for the phase adjustdriver. Therefore, in the above example, selecting the error inputsignal of the phase adjust driver to the level driven by the I dc outputfrom the DUT gives the desired output on the Q channel. A monitorselector, such as the monitor selector 236 of FIG. 2 controls which ofthe monitored channel outputs of the DUT is provided as the error inputof the phase adjust driver to cause the phase adjust loop to shift thephase of the LO input relative to the Signal input of the DUT.

If the DC monitor signals are available from the DUT, but phase lockingis not sufficiently robust, matching the phase of the Signal to the LOcan be done in software, as illustrated in FIG. 3. The testing setup ofFIG. 3 is similar to that of FIG. 2, except a digitizing oscilloscope360 is coupled to a DUT 320. No phase adjust loop is driven in thisembodiment, and instead the DC monitor signals from the DUT 320 are fedto the digitizing oscilloscope 360. The digitizing oscilloscope 360interprets the desired DC monitor signal and informs a performanceoscilloscope 350 a phase adjust amount for precisely aligning thesignals received at the LO and Signal inputs.

Since sometimes even the DC monitor outputs from the DUT are notavailable, an alternative method to adjust the Signal relative to the LOcan be used to determine the appropriate phase correction between theDUT and reference paths, as illustrated in FIG. 4.

In FIG. 4, the beat frequency between the laser 410 output at thereference detector 416 and the laser 410 output read at the DUT 420 willbe available at an operational frequency of an Acoustic Optic Modulator(AOM) 431, such as approximately 80 MHz. This can be separated out fromthe DUT 420 response using a digital filter 451 in the oscilloscope 450.During subsequent analysis, the phase of the 80 MHz signal is subtractedfrom the measured DUT 420 rf signal to correct for phase wander betweenthe two paths.

In these cases, the fields at the DUT are as follows (neglecting lossesfor simplicity):

=E ₁ e ^(jω) ¹ ^(t) +E ₂ e ^(jω) ² ^(t) =E ₁₀ e ^(jθ) ¹ e ^(jω) ¹ ^(t)+E ₂₀ e ^(jθ) ² e ^(jω) ² ^(t)  Signal Field:

=E ₂ e ^(jω) ² ^(t) e ^(jφ) =E ₂₀ e ^(jθ) ² e ^(jω) ² ^(t) e ^(jδωt) e^(jφ)  LO Field:The E*E product that occurs in the DUT is then:

v_(DUT) = H_(DUT)(ω) = H_(DUT)(ω₁ − ω₂ − δω)E₁₀E₂₀e^(j θ₁ − j θ₂ − j φ)e^(j ω₁t − j ω₂t − j δω t) + H_(DUT)(δω)E₂₀²e^(−j φ)e^(−j δω t)And the real part (taking H_(DUT)(δω)=1)

v_(DUT) = Re[H_(DUT)(ω)] = H_(DUT)(ω₁ − ω₂ − δω)E₁₀E₂₀cos [(ω₁ − ω₂ − δω)t + (θ₁ − θ₂) − φ + θ_(DUT)] + E₂₀²cos (δω t + φ)

While the inter-path phase can be determined from the second term bycomparing with the drive signal for the AOM 470, which is generated by asignal generator 472 and coupled to the AOM by an RF coupler 474, thisis not strictly necessary to extract the DUT phase. It is not necessaryto measure the AOM drive signal unless the inter-path phase is ofspecific interest.

The field at the reference detector 416 is directly proportional to theSignal field, assuming a perfectly deskewed system where samplinginstants of the oscilloscope 450 occur such that there is no phase delaydifference between the Signal and reference detector paths. In thiscase, the voltage at the reference detector output is:ν_(REF)=Re[H _(REF)(ω)

]=|H _(REF)(ω₁−ω₂)|E ₁₀ ² cos [(ω₁−ω₂)t+(θ₁−θ₂)+θ_(REF)]

Again, assuming a deskewed system so that “t” in both equations can betaken as the same time axis, the DUT output at ω₁−ω₂+δω can now becompared to the reference detector amplitude and phase to determineH_(DUT)(ω₁−ω₂+δω) relative to H_(REF)(ω₁−ω₂) and θ_(DUT) relative toθ_(REF). This can be done even in the presence of fluctuations in φwhich are expected due to the separate signal path. The correction ismade easier by keeping the various path lengths approximately equal fromlaser 410 to the three optical inputs.

Since the DUT 420 and reference detector 416 see slightly differentfrequencies, it is also important to determine the response of thereference detector 416 at δω offset from the test frequency, ω₁−ω₂,relative to its response at the test frequency. If the referencedetector has been fully characterized, this requirement should not posea significant difficulty since only the relative values are needed. Theimpact of this requirement can be reduced by lowering the modulationfrequency δω. While 80 MHz is a typical value for an acousto-opticmodulator, much lower frequencies are possible using serrodyne orsingle-side-band modulation techniques.

An example method of phase comparison is demonstrated with reference toFIGS. 5A, 5B, and FIG. 6. The signal at δω is separated from the DUT 420signal with a low-pass filter, such as the filter 510 illustrated inFIG. 5A. This is then mixed with the DUT signal and filtered to reachthe reference detector frequency, ω₁−ω₂. The sum term is desired both tocompare signals of equal frequency as well as to get the proper sign tosubtract the inter-path phase wander. After filtering, the phase isobtained by comparing to the reference detector signal using thearc-cosine of the dot-product, as shown above. In other words, thebandpass filter 510 is applied to the DUT 420 signal to extract thesum-frequency mixer term 520, as well as the input spectra 530 and theoutput signal spectra 540, as illustrated in FIG. 5B. The system maythen be deskewed by plotting phase vs. test frequency to find theta vs.omega as illustrated in FIG. 6. This slope is the skew, which may becompensated to get the phase response of the DUT 420. In FIG. 6, thephase vs. frequency data before deskew is illustrated as the slopingline 610, and as the flat line 620 after deskew. The fact that therandomly fluctuating frequencies provide phase along a straight linethat is reproducible proves that the phase fluctuations are removedsuccessfully.

With reference still to FIG. 6, the skew computed from the slope of thephase/frequency graph is 952 ps. Adding this delay to the RefDet path(which was the shorter one) by introducing a Deskew value of −952 ps inthe oscilloscope Deskew UI for that channel, gave repeatable phasevalues independent of frequency.

Alternatively, the signals v_(ref) and v_(DUT) can be sampled and storedwith, for example, a digital oscilloscope. The frequency and phases thatappear can be extracted from these stored data using any of a variety ofmathematical techniques. One illustrative technique extracts theparameters by the least-squares fitting of the stored data to onegeneral sinusoid (for v_(ref)) or the sum of two general sinusoids (forv_(DUT)) at different frequencies. This procedure, in particular,provides a value for the phase difference φ arising from the fiber pathdifferences. This phase difference can then be removed to obtain thetrue phase response of the DUT. Generally, the fit of the data to thesinusoids is a nonlinear minimization problem, where an iterativetechnique must be used. A Fast Fourier Transform (FFT) of the voltagedata usually provides good initial values for the sinusoid frequencies.

FIG. 7 is a block and schematic diagram illustrating an example phaseadjust driver according to embodiments of the invention. In thisexample, current monitor outputs are available from the DUT, such as thecurrent monitors 182, 184 of FIG. 1. A monitor selector 736 selects theparticular desired outputs from the DUT, which may be a pair ofdifferential photocurrent outputs as illustrated in FIG. 1. The monitorselector 736 passes the selected outputs to a phase adjust driver 700,which may be an embodiment of the phase adjust driver 232 of FIG. 2. Thephase adjust driver 700 may include several sections, such as a polarityswitching section, a P−N subtraction section, and a section to performloop filtering and gain.

In operation, the monitor selector 736 passes the selected photocurrentmonitor outputs to a polarity switch 710 component of the phase adjustdriver 700. The polarity switch 710 allows a user to change the polarityof the error signal, and therefore gives the user control to selectwhich one of the pair of differential output diodes of the selectedchannel of the DUT will be tested.

The differential inputs, after the desired polarity is chosen by thepolarity switch 710, are presented to a differential amplifier 720,which generates a voltage signal indicative of the difference of itsinput in the P−N subtractor section of the phase adjust driver 700. Theoutput of the differential amplifier 720 is passed to another amplifier730, which is in the loop filtering and gain section of the phase adjustdriver 700. The output of the amplifier 730 is the output of the phaseadjust driver 700 that drives the phase adjust loop 232 of FIG. 2,although, in some embodiments, the signal from the amplifier 730 may gothrough yet another amplifier or series of amplifiers to generate enoughgain to drive the phase adjust loop 232.

In operation, the phase adjust driver 700 creates an output signal froman input, which itself is one of the monitored photocurrents of the DUT,for driving the phase adjust loop 232. The output signal of the phaseadjust driver 700 keeps the phase of the LO input extremely preciselyaligned with its desired position relative to the Signal input of theDUT. Further, the phase adjust driver 700 allows selection of differentrelative phases simply by changing which of the monitored photocurrents,of the DUT are chosen to be the selected inputs to the phase adjustdriver 700. Plus, the polarity switch 710 allows the user to selectwhich particular photodiode, P or N, of the differential pair ofphotodiodes is selected.

It shall be well understood to a person skilled in the art that theinvention is not limited to any particular standard, but is applicableto systems having similar architecture without depraving from theinventive scope.

The foregoing description has been described for purposes of clarity andunderstanding. In view of the wide variety of permutations to theembodiments described herein, the description is intended to beillustrative only, and should not be taken as limiting the scope of theinvention. Although specific embodiments of the invention have beenillustrated and described for purposes of illustration, variousmodifications may be made without departing from the spirit and scope ofthe invention.

What is claimed is:
 1. A system for determining a phase angle of a localoscillator relative to a phase angle of a signal input of a Device UnderTest (DUT), comprising: a first laser source structured to generate thesignal input for the DUT, and a signal input for a reference detector; asecond laser source structured to generate a Local Oscillator input forthe DUT; a phase maintaining laser coupler having two inputs and twooutputs, the first input to the coupler coupled to the output of thefirst laser source, the second input to the coupler coupled to theoutput of the second laser source, the first output to the couplercoupled to the reference detector, and the second output to the couplercoupled to the DUT; a current monitor structured to monitor a selectedone of a plurality of outputs of the DUT, the plurality of outputs ofthe DUT including in-phase and quadrature outputs for each of an X and aY channel; and a phase measurer coupled to the output of the DUTselected by the current monitor, and structured to measure an erroramount by which the LO is out of phase from the signal input to the DUT.2. The system for determining a phase angle of a local oscillatorrelative to a phase angle of a signal input of a Device Under Test (DUT)according to claim 1, in which the phase measurer is a DC gain loop, andfurther comprising a phase adjuster coupled to the phase measurer andstructured to modify a phase of the LO by an amount related to theoutput of the phase measurer.
 3. The system for determining a phaseangle of a local oscillator relative to a phase angle of a signal inputof a Device Under Test (DUT) according to claim 2, in which the phaseadjuster is a fiber loop stretcher.
 4. The system for determining aphase angle of a local oscillator relative to a phase angle of a signalinput of a Device Under Test (DUT) according to claim 2, in which thephase adjuster causes a phase angle difference of the signal input tothe DUT and the LO input to the DUT to be controllably driven toapproximately zero.
 5. The system for determining a phase angle of alocal oscillator relative to a phase angle of a signal input of a DeviceUnder Test (DUT) according to claim 1, in which the phase measurer is afirst oscilloscope coupled to the current monitor.
 6. The system fordetermining a phase angle of a local oscillator relative to a phaseangle of a signal input of a Device Under Test (DUT) according to claim5 in which the first oscilloscope passes a measurement signal to asecond oscilloscope.
 7. A system for determining a phase angle of alocal oscillator relative to a phase angle of a signal input of a DeviceUnder Test (DUT), comprising: a first laser source structured togenerate the signal input for the DUT, and a signal input for areference detector; a second laser source structured to generate a LocalOscillator input for the DUT; a phase maintaining laser coupler havingtwo inputs and two outputs, the first input to the coupler coupled tothe output of the first laser source, the second input to the couplercoupled to the output of the second laser source, the first output tothe coupler coupled to the reference detector, and the second output tothe coupler coupled to the DUT; an Acoustic Optic Modulator (AOM)coupled between the second laser source and the LO input to the DUT; inwhich a beat frequency between the signal input for the referencedetector and the signal input to the DUT approximates an operationalfrequency of the AOM.
 8. The system for determining a phase angle of alocal oscillator relative to a phase angle of a signal input of a DeviceUnder Test (DUT) according to claim 7, further comprising an RF couplerstructured to pass the beat frequency to an oscilloscope coupled to theDUT and to the reference detector.
 9. The system for determining a phaseangle of a local oscillator relative to a phase angle of a signal inputof a Device Under Test (DUT) according to claim 7, in which theoperational frequency of the AOM is less than 100 MHz.
 10. The systemfor determining a phase angle of a local oscillator relative to a phaseangle of a signal input of a Device Under Test (DUT) according to claim7, further comprising a means for sampling and storing the voltagesoutput by the DUT, in which the stored voltage data is then used todetermine the desired relative phase angle.
 11. The system fordetermining a phase angle of a local oscillator relative to a phaseangle of a signal input of a Device Under Test (DUT) according to claim10, in which the means for sampling and storing the voltage data isprovided by a digital storage oscilloscope.
 12. The system fordetermining a phase angle of a local oscillator relative to a phaseangle of a signal input of a Device Under Test (DUT) according to claim10, where the desired relative phase angle is obtained from the storedvoltage data by least-square fitting to one general sinusoid, or the sumof general sinusoids at different frequencies.
 13. A method fordetermining a phase angle of a local oscillator relative to a phaseangle of a signal input of a Device Under Test (DUT), the methodcomprising: generating a first laser signal at a first laser, the firstlaser signal to connect to the DUT; generating a second laser signal ata second laser, the second laser signal connected to a local oscillatorinput to the DUT; coupling a phase of the first laser signal to a phaseof the second laser signal in a phase maintaining coupler; measuring aphase error between the first laser signal and the second laser signal;and compensating for the measured phase error.
 14. The method fordetermining a phase angle of a local oscillator relative to a phaseangle of a signal input of a Device Under Test (DUT) according to claim13, in which compensating for the measured phase error comprisesphysically adjusting a fiber length of a fiber carrying the first lasersignal or a fiber carrying the second laser signal.
 15. The method fordetermining a phase angle of a local oscillator relative to a phaseangle of a signal input of a Device Under Test (DUT) according to claim13, in which physically adjusting a fiber length comprises stretching aloop of fiber.
 16. The method for determining a phase angle of a localoscillator relative to a phase angle of a signal input of a Device UnderTest (DUT) according to claim 13, in which compensating for the measuredphase error comprises sending the measured phase error to anoscilloscope coupled to the output of the DUT, and compensating for themeasured phase error in the oscilloscope.
 17. The method for determininga phase angle of a local oscillator relative to a phase angle of asignal input of a Device Under Test (DUT) according to claim 16 in whichsending the measured phase error to an oscilloscope coupled to theoutput of the DUT comprises sending the measured phase error from asecond oscilloscope.
 18. The method for determining a phase angle of alocal oscillator relative to a phase angle of a signal input of a DeviceUnder Test (DUT) according to claim 16 in which an Acoustic OpticModulator (AOM) is coupled between the second laser and the DUT, and inwhich sending the measured phase error to an oscilloscope coupled to theoutput of the DUT comprises sending the operating frequency of the AOM.